Strong Acid Applied in HPHT Wells

Abstract Historically, our company has limited the use of acids at very high temperatures (>140 °C) to relatively weak organic acids. This limitation stems from corrosivity of well materials and various acid + corrosion inhibitor (CI) blends have therefore been investigated through the years. Weak acids do dissolve CaCO3, but we have experienced problems using them to regain productivity in wells with substantial amounts of scale. It is believed that the reaction rate is simply too low to dissolve enough scale within the timeframe of a typical soaking operation. A restriction at older gas/condensate fields is often also the low reservoir pressure. Hence to be able to start a well after an operation, one could be limited to small volumes and have little control of placement and therefore time that acid is in contact with the solids. A strong acid would offer much higher reaction rates and would presumably be more successful. The aim of the present work was to check whether a commercially available strong acid blend could be used at >140 °C. The acid blend was based on methane sulphonic acid (MSA) with a corrosion inhibitor package. Herein we focus on three aspects of the qualification process; Laboratory tests for corrosion rate, assessment of formation damage and the final step of field-use. Regarding corrosion, it was chosen to use a criterion of acceptable corrosion rate with 5 vol% of 8 wt% NaCl brine mixed into the acid. The acid was qualified for use up to 152 °C in contact with 13CRS-110 tubing material for 6 hours. The same acid, combined with two different corrosion inhibitor (CI) packages, has been used in eight well operations, where three of them were at >140 °C. Five were successful, one showed little change, while in two cases the rates (injection or production) after acid exposure were lower than before. One of the failures was attributed to unintentional stimulation of a water producing interval, while the underlying reason for the other remains unknown. These failures were not believed to be related to issues with the acid blend itself, and therefore, the acid blend is regarded as suitable for further field use.